According to the Oxford English Dictionary Online, both “relevance” and “relevancy” are nouns meaning “The quality or fact of being relevant.”

The suffix -ancy is “A modern Eng. differentiated form of the earlier -ANCE, expressing more distinctly the sense of quality, state, or condition…as distinct from the sense of action or process.”

So it’s really up to the person using the word which suffix to use. Besides, Grammar Naziness is so overrated, as long as the idea the person is trying to communicate is successfully conveyed. In this case, it is. 🙂